1. Field of the Invention
This invention relates to anti-corrosive paints for metals. More particularly, this relates to paints containing conjugated polymers as corrosion preventive agents.
2. Prior Art
Corrosion inhibiting primers for metals such as steel are known. Corrosion inhibiting primers generally fall within two broad categories: those which provide high barrier coatings and those which impart active corrosion inhibiting properties. The high barrier coatings include various epoxies, alkyds, polyurethanes and the like. Among the latter are zinc-rich coatings which provide cathodic protection. In a middle ground are red lead and chromates which are thought to provide active protection but also function to enhance barrier properties by neutralizing acids and insolubilizing salts. These corrosion inhibiting pigments which have been, for years, the work horses of the industry, are now considered distinctly non-preferred because of their adverse toxicological properties. As a result, the industry is in search of corrosion inhibitors which perform well and are environmentally acceptable. Paints which contain conjugated backbone polymers provide such an alternative.
Various mentions have been made of the possible use of conductive conjugated polymers as corrosion inhibitors. Essentially, two different proposals have been previously made regarding the possible working mechanism for conjugated conducting polymers.
The first of these possible mechanisms falls into the category of anodic passivation of the substrate metal. This mechanism is applicable to selected metals, which include steel and aluminum, which display anodic polarization curves which have a peak in the corrosion current followed by a region of low (but non-zero) current as the potential is swept in the anodic (oxidative) direction from the rest potential (corrosion potential) of the metal in question. The region of low current is commonly referred to as the anodic passivation region. In order for the conducting polymer to function in this manner, it must be in at least a partially oxidized (p-type) state; it must be conductive (or at least semi conductive); and it must be in electrical contact with the coated metal. This idea has seen little practical application, especially in terms of a coating system designed to achieve this effect. The idea for employing conducting polymers to this end was first advanced by Colman Brian of NASA, and was pursued by A. MacDiarmid and Naseer Ahmad at the University of Pennsylvania (unpublished). Investigations to date have entailed only the use of pure conductive polymer coatings (electrochemically deposited or deposited from solution), and have provided little evidence of active corrosion protection.
The second proposed mechanism relies on the formation of a metal-semiconductor (MS) or a metal-insulator-semiconductor (MIS) junction between the base metal and a semiconductive coating. This method for corrosion protection has been demonstrated for thin coatings of wide-band-gap semiconductors (e.g., indium-doped tin oxide) which have been deposited on a metal surface. (F. L. Jain et al. "Corrosion Prevention in Metals using Layered Semiconductor/Insulator Structures Forming an Interfacial Electronic Barrier" in Adhesives, Sealants, and Coatings for Harsh Space Environments, Polym. Sci. and Tech., Vol. 37, PP. 381-404, (Plenum, 1988). This method relies on the formation of an electronic barrier which limits the transfer of electrons from the metal to oxidants (e.g., O.sub.2) in the environment. Such a mechanism requires that the coating be semiconductive (non-degenerate) so that band bending occurs at the interface and establishes a region of depleted charge carrier concentration and an electronic barrier commonly referred to as a built-in potential. It is necessary that this semiconductive coating be applied directly to the metal (MIS junction) or on top of an insulating layer on the metal which is not thicker than about 200A (MIS junction). It has been proposed that various semiconductive polymers and organic complexes including polyacetylene and phthalocyanine might be used for this purpose (Jain et al.). The semiconductive polymer may be p-type or n-type; p-type is preferred.
All but one of the references and previous disclosures of the potential use of conjugated backbone (conductive) polymers as corrosion inhibitors refer to the use of continuous coatings of neat conductive polymers produced by solution coating techniques or by electropolymerization directly on the substrate metal. See for example, L. E. A. Berlouis and D. J. Schiffrin, "Recent Advances in Electrochemical Polymerization for Surface Coating", Trans IMF, Vol 64, p42 (1986) and "Anodic Synthesis of Polyaniline Coatings onto Fe Sheets", G. Mengoli, et al., J. Appl. Polym. Sci., 26, 4247 (1981).
The major exception to using only the neat conductive polymer is PCT 88/00798 which is directed to an intrinsically conductive polymer that exists in the form of a dispersible solid composed of primary particles having a specific surface area according to BET of &gt;15 M.sup.2 /g and a weight average diameter when dispersed of less than 500 nm. It is disclosed that these particles are particularly useful for further processing as a dispersed phase in polymer-based paints for corrosion protection as shown in Example 18 of PCT 88/00798. The report "Conductive Organic Polymers as Corrosion Control Coatings," NASA Technical Memorandum 103811, pp. 3-5, (1990) also describes composites of conjugated polymers with epoxies.
PCT-WO 89/01694 and PCT-WO 90/10297 describe blends of polyaniline and one or more thermosetting and thermoplastic resins.